Coil form



OCf- 1, 1963 L E ROY F. MGFARLANE 3,105,948

COIL FORM Filed Nov. 13,

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United vStatesY Patent O 3,105,948 CGILFORM v Le Roy F. McFarlane, RR.1, St. Charles rIOWnship,

Kane County, lll. Filed Nov. 13, 1961, Ser. No. 151,714

8 Claims. (Cl. 336-136) The present invention pertains to improvementsin variable inductors and is particularly concerned with theconstruction and assembly of a coil form embodying novel resilientlycompressible means adapting the coil form for the reception and fixedadjustment of a core therein.

i Coil forms of the type herein disclosed are primarily `intended foruse in the production of inductors having various types andkinds ofwindings which when connected in the specific circuitry, such as radioand television circuits, require critical adjustment electrically to agiven frequency while current is owing therethrough. Final alignment ofthe inductor is accomplished by providing the coil form with an ironcore which can be screwed axially therein to establish the necessaryfinal electrical frequency required for satisfactory operation.

Although the coil form may be made of any suitable tubular shapedmaterial capable of withstanding high mechanical stresses withoutdeformation and-which is resistant to moisture absorption and fungusgrowth, applicant has found that a satisfactory coil form or tube madeof alpha cellulose paper spirally or convolutely wound and impregnatedunder vacuum with polyester resin or, one that is molded or built up byspray or lay-up processes upon a basic wire screen frame, has all therequiste characteristics. The resulting coil form or tube is a rigidhard body having an extremely low coefficient of expansion and one thatmay be slotted, stapled or drilled without fracture.

The coil form or tube of the present invention is formed on its insidesurface with a series, in this instance three, of resilient spirallyarranged ribs or ridges that are securely bonded to the tube. Theseridges, formed in either clockwise or counterclockwise directionsrelative to the tube, afford means to frictionally and mechanicallyreceive and hold the threaded core in any fixed position of adjustment.To fulfillV such purpose, said ridges have substantially coarser pitchthan that of the threads of ythe core and the smallest diameter affordedby the presence of the ridges is less than the outside diameter of thecore. This permits the core, Whose threads will extend in crossingrelation with respect to said ridges upon core movement into the tube,to generate its own thread in the ridges by material displacement asdistinguished from cutting. It is therefore another advantage of thepresent invention to provide a coil form with a resiliently compressibleliner capable of material displacement to accommodate the threads of acore threaded thereinto and in a manner to provide mechanical resistanceto unintended rotation of the core or axial displacement thereof.Obviously, the amount of torque required to rotate the core can bevaried in different coil forms at Vthektime of manufacture by varyingthe resiliency, width, spacing,

number of ridges, and/or thickness of the ridges.

The structure, by means of which the above noted and other advantages ofthe invention are attained, will be described in the followingspecification, taken in conjunction with the accompanying drawings,showing a preferred illustrative embodiment of the invention, in which:

FIG. l is a perspective View of a variable inductor embodying'theinvention;

FIG. 2 is an end View, on an enlarged scale, of the coil form;

FIG. 3 is a fragmentary enlarged diametrical sectional view of the coilform;

FIG. 4 is a view similar to FIG. y3, showing the core threadedthereinto; and,

FIG. 5 is a view of the coil formy split and laid flat to illustrate arepresentative pattern of ridge spacing.

ln the accompanying drawings, FIG. 1 illustrates an exemplary type ofvariable inductor 10 which comprises a coil form 11 having a pair oflongitudinally spacedcoils 12 wound thereon and provided with afrequency adjusting core 13 adapting the coils to be adjustedelectrically to a given frequency. Inductors of the type illustratedfrequently are enclosed in a metal shield (notshown) to minimizeinterference with component parts of the electric circuitry iny whichthe inductor is installed.

The coil form 11 is tubular and it is made from suitable dielectricmaterial having shape retaining characteristics adapting it to resisthigh mechanical stress so as to resist fracture and deformation. Thebasic material of the coil form may be paper, asbestos, cellulosefibers, or other stress resisting material preferably impregnated ,withany suitable thermosetting or thermo-plastic material such for exampleas polyester, epoxy, melamine, phenolic and the like, to make itimpervious to moisture and fungus growth. The coilA form 11 may bemolded or otherwise formed tubular and it may be centerless-ground toprovide a smooth outside surface forcoil winding.

Thecoil form lil is provided with a novel structure integrally attachedto its inside circumferential surface to afford mechanical andfrictional means for receiving a threaded core therein and holding it ina critical position of adjustment. This is best accomplished byproviding the inside surface of the coil form 11 with a plurality ofspiral ridges or ribs 14 having coarse pitch (see FIGS. 3 and 4). In thepresent disclosure applicant has provided three such ribs equally spacedapart circumferentially and having parallel convolutions and preferablyextending throughout the length of the coil form. Clearly, the ribs may,extend either in clockwise .or counter-clockwise directions in the coilform.k It should be appreciated that while three ribs are disclosed, afewer or greater number can be used and that, under certain userequirements, the entire inside circumferential surface of coil form 11may be coated with a material responding to the material of the ribs.Also, the ribsV may be formed as interrupted ridges or the insidesurface `of the coil form may have random or pattern oriented nodulesformed thereon.

The material of the ribs or inside coating may comprise any required.substance having the necessary cold How characteristics rendering theribs resiliently deformable under compression but at the same timeresistant to permanent displacement under the action of the core steel,or other commonly used materials andit is externally threaded, as at 15,and provided at one end with a .tool recess 16 adapting it to be engagedfor threading it into one end of coil form 11. The largest diameter ofthe threaded core is greater than the smallestk diameter of ridges 14and less than the inside diameter of coil form l1." Accordingly, whenthe vcore is threaded into the coil form, its threads extend or cutacross said ridges, and thus the core generates its own threads in theribs by compressive displacement of the resilient material thereof.

The resiliency of this material, combined with the thicki ness, Widthand number of ribs combine to provide a strong mechanical resistance torotation of the core. The amount of torque required to rotate the corecan be varied by amasar;

varying the resiliency, width and/ or thickness of the ribs or surfacecoating, or in instances of the spiral ribs, by varying the pitch andnumber thereof. The core is accordingly compressibly gripped and heldfirmly by both mechanical and frictional forces against even slightdisplacement which, if permitted, would result in nonalignment of thefrequencies sought to be aligned. By way of example, the torque requiredto rotate the core preferably falls within the range of .25 in. oz. to20 in. oz., depending upon the physical size of the inductor and itsintended use. The resilient material engagement effected by the hereindisclosed structure also provides high resistance to a force appliedaxially which would tend to push the core into or out of the coil formand thereby cause electrical misalignment.

Because of the resiliency of the material engaged by the threads of thecore and the rigidity of coil form 1i, the completed assembly is capableof maintaining adequate torque through many adjustments of the core.A

Should it happen that the torque required to turn the core drops below apredetermined minimum, as a result of repeated cycling, the core may beremoved and reinserted at a slightly different starting point radiallyrelative to the entrance end of the coil form. This will restore theoriginal torque characteristics because the crests of the core threadswill engage with different areas of the resilient material.

Although I have described in considerable detail, a preferred embodimentof my invention, it will be understood that the description thereof isintended to be illustrative, rather than restrictive, as many details ofthe structure may be modified or changed without departing from thespirit or scope of the invention. Accordingly, I do not desire to berestricted to the exact construction described.

What I claim and desire to secure by Letters Patent of the United Statesis:

1. A variable inductor comprising a hard distortion resistant tubularcoil form, at least one coil wound on said coil form, at least oneresiliently deformable rib spirally arranged on and projecting inwardlyfrom the inside surface of said coil form, and a core threaded into saidcoil form, said rib having substantially coarser pitch than the threadsof said core and said core having an outside diameter greater than theinside diameter defined by said rib whereby the threads of the core cutacross said rib and in so doing cause compressive displacement of thematerial of the ribs engaged by the threads of said core.

2. A variable inductor according to claim 1, wherein said rib is of adifferent material than that of the coil form and is bonded to theinside surface of said coil form.

3. A variable inductor according to claim 1, wherein said rib iscomposed of a material which is resistant to permanent displacementunder the action of the core threads thereon.

4. A variable inductor according to claim 1, wherein a plurality of saidribs spaced circumferentially about the p eiplhery of the inside surfaceof said coil form are prov1 e 5. A variable inductor according to claim1, wherein the aforesaid engagement of said threads and said rib providethe sole means of securing the core ina predetermined axial positionwithin the tube.

6. As a new article of manufacture, a coil form cornprising a tubularmember of stress-resistant material having at least one spirallyarranged rib projecting inwardly from its inside surface and definingthread-engaging means for a threaded core, said rib having substantiallycoarser pitch than the threads of said core and being of a materialwhich is resiliently deformable by the threads of said core but beingsufficiently resistant to such deformation that said rib is capable ofsecuring the core in a predetermined axial position within said coilform solely by interengagement of the core threads with said rib.

7. A coil form according to claim 6, wherein said rib is of a differentmaterial than that of the coil form and is bonded to the inside surfaceof said coil form.

8. A coil form according to claim 7, wherein a plurality of said ribsspaced circumferentially about the periphery of the inside surface ofsaid coil form are provided.

References Cited in the file of this patent vUNTTED STATES PATENTS2,462,822 Wood Feb. 22, 1949 2,584,723 Mackey Feb. 5, 1952 2,838,738VAntalek et al June 10, 1958 2,995,719 Osborn et al Aug. 8, 1961

1. A VARIABLE INDUCTOR COMPRISING A HARD DISTORTION RESISTANT TUBULARCOIL FORM, AT LEAST ONE COIL WOUND ON SAID COIL FORM, AT LEAST ONERESILIENTLY DEFORMABLE RIB SPIRALLY ARRANGED ON AND PROJECTING INWARDLYFROM THE INSIDE SURFACE OF SAID COIL FORM, AND A CORE THREADED INTO SAIDCOIL FORM, SAID RIB HAVING SUBSTANTIALLY COARSER PITCH THAN THE THREADSOF SAID CORE AND SAID CORE HAVING AN OUTSIDE DIAMETER GREATER THAN THEINSIDE DIAMETER DEFINED BY SAID RIB WHEREBY THE THREADS OF THE CORE CUTACROSS SAID RIB AND IN SO DOING CAUSE COMPRESSIVE DISPLACEMENT OF THEMATERIAL OF THE RIBS ENGAGED BY THE THREADS OF SAID CORE.